He is my friend. He is working with me.
He is my friend Who is working with me.

Here is Who is joined the two sentences into one.

We will see some more examples.

I met the person Who attended our school function.
I caught the person Who stolen my purse.

I lost the book which I bought yesterday.
I got the key which I lost yerterday.

This is the novel that I read manytimes.
I have lost the pen that she gave me.